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Friday, April 29, 2022

Is the temperature reading of your TPMS correct? Probably Yes, BUT...

 IMO TPMS are of little value when it comes to temperature warning for tires. Same for IR guns.

I have written a number of times on TPMS Temperature reading really being a reading of the metal wheel and not the rubber tire as metal conducts heat while rubber is an excellent insulator. So your TPMS can be used to provide warning of potential problems with wheel bearings, breaks and other metal mechanical parts.

These posts use data from my ongoing, direct comparison of internal vs external sensor TPMS. My data suggests that there is no meaningful difference in pressure reading based on the test of 12 sensors.
When seeing people claim the TPMS temperature reading were useful for predicting impending tire failure, the engineering DNA in me kicked in and I devised a plan to test 12 sensors. These come from two different companies. One set of 6 external sensors is from TireTraker and one set of 6 internal sensors is from TechnoRV who provided the internal TST  system. My thanks to both companies for their support in my efforts to help educate the RV community about tires.

The question is: How do I make the test both fair and useful? For the pressure test, I decided to eliminate as many variables as possible and get all 12 readings from the same air chamber at the same time and compare them all against my personal digital hand gauge that I have checked against an ISO certified laboratory gauge.
Note: my hand gauge reads to 0.5 psi which is way more precise than anyone needs for checking tires in normal highway use.

Here is the test fixture I made.
6 Internal sensors are inside the tube and 6 external sensors are on the outside along with a pressure regulator, a safety pop-off, and a reference dial gauge that allowed a visual check plus the test port for my hand held digital gauge that I have confirmed accurate to +/- 0.5 psi against an ISO Certified master gauge.

Here are the results of my comparison test. The target pressure is 80.0 psi as reported by my handheld digital gauge.

Set A  1 reading of 78 psi,  5 readings of 79 psi
Set B  2 readings of 78, 2 readings of 79 and 2 readings of 80 psi

I also recorded the temperature.
Set A  4 readings of 66 F, one each of 64 and 68F
Set B  4 readings of 69 F  and 2 readings of 68

I have recorded the internal vs external pressure difference in reported running pressure with the following results.

I do not consider any of the differences in the readings of the internal vs external sensors pressure to be significant or meaningful for a TPMS, remembering that at Ambient I observed 3 to 5 psi differences when all 12 sensors were measuring an identical pressure, which is not too far off the claimed accuracy for TPMS.

The problem with TPMS temperature readings is that they are not able to read the hot spot of a tire. Here is a graphic representation to tire temperature differences.

Since heat can kill a tire, and tires simply do not fail based on their average temperature but they can fail if a single spot exceeds the ability of the rubber to maintain its strength. With this large temperature spread and the fact that the hottest spot in a radial tire is about 3/8" to 1/2" away from the air chamber and the internal TPM sensor is reading the average air temperature in the air chamber, I would ask why anyone would believe that a TPMS "High Temperature" reading is a reliable method of warning of an impending tire failure.

I started this post by saying I did not think that TPMS or IR guns were reliable tools for predicting an impending tire failure based on the reported temperature.  I will cover IR guns next week.


Friday, April 22, 2022

Simple question: Why do tires fail?

In reality, tires fail for a relatively small number of Root Cause reasons that can be discovered, but it does require detailed and sometimes exhaustive investigation. By "Root Cause," I mean the initiating feature or condition that eventually led to the failure. Too often people confuse the tire's ending condition with the initiating or "Root Cause" reason for the tire to end up in the condition they are observing.

For the last few years of my 40-year career as a tire engineer, my primary job duty was to investigate failed tires that had conditions that were hard to understand or provided confusing and sometimes contradictory evidence as to the Root Cause for their condition. With that background, I have developed some guiding principles for the most likely reasons for tires to fail.

These are, in no special order:
1. External Damage. These include punctures, cuts, impacts, wheel and valve failures and similar conditions.
2. Run Low Flex Failure. This is best described by the failure of the tire sidewall due to excessive flexing. The flexing is the result of operation when the tire is significantly under-inflated.
3. Belt & Tread Separations. This is when the belts of a radial tire detach from the body or from each other and/or when the tread rubber detaches from the top belt.
4. Manufacturing Defect. For the purpose of this discussion this would be when components of a tire were not manufactured to intended specification.

The "Why" for some of these are obvious, such as the External Damage category.
Others can be further traced to various contributory reasons.

Run Low Flex Failure This is a more accurate description of what some incorrectly call "Blowout". When a tire looses significant inflation air they flex much more that the body cord can tolerate. Excess heat is generated which can, in extreme cases, result in the Polyester actually melting. I showed examples of what this can look link in THIS post. Polyester is used in most Passenger, ST type and LT type tires. Steel body tires are used on most Class-A RVs, the steel can fatigue. I covered "Zipper" failures and steel fatigue in THIS post.

Belt & Tread Separations occur primarily because the rubber around the belt cords or between the belt rubber and the tread rubber isn't strong enough. Now it can be weak for a variety of reasons. Some might be design, some might be manufacturing and some weakening can be caused by cumulative improper service conditions.

Now let's talk about tires made by a reputable company, i.e., one that has tire stores and dealers with physical stores and sells tires that have a warranty of two or more years.
If this tire is subjected to thousands of miles of lower inflation or higher loading or operation at higher speeds or stored is full sunlight it is reasonable to expect the strength of the belt & tread area rubber to loose a good portion of its strength primarily due to the increased operating temperature of the tire. If this tire is in service on a tandem axle trailer then there is also increased Interply Shear which can overload the belt area rubber. The combination of the above may result in a belt or tread separation.

Another possibility is a Manufacturing Defect. These usually occur in small numbers as tires are built in batches so the substitution of the wrong type of rubber may cause tires to fail. It is important to understand that in almost all cases this type of "defect" usually shows up at early life of the tire. It is also very important to understand that unlike some lawyers, engineers deal in facts and logic. Simply having a tire fail is not in it self proof of a defect but all to often that is the position that those in the legal profession seem to jump to. That approach may result in a nice payout to the lawyer but will not result in product improvement as there has been no determination or identification of the actual initiating "defect".

Finally there is the possibility of a design weakness. Now I do not believe that anyone is intentionally designing tires to fail but this sometimes may occur when the performance goals of the manufacturer are limited to meet the bare minimum for strength and durability and the focus is primarily on low cost. There is no absolute way to identify these tires but I do believe there are indicators when looking for tires that are made to a higher standard of quality and durability.
I would consider a tire company reputable if it had a chain of stores across the country. If the tires carry the name of the manufacturer and have a multi-year warranty the longer the better then they probably have higher durability requirements than those established by DOT.

 For ST type tires I would look for tires with a Speed Symbol of "L" (75 mph) or SLOWER. Now a side point about speed. I did a POST some time focusing on speed and strongly recommend you read it. I do not think I would recommend any tire for general use that does not have a speed rating molded on the sidewall. Few people realize that ST tire loading is based on the assumption of 65 mph Max operation speed

I do hope this post will help some to have a better understanding of what can contribute to tire failure and consider what you can do to lower the chances of having such a failure. 



Thursday, April 14, 2022

I just had a tire failure, for no reason

If you follow any RV forums you probably see a statement like this every few weeks. I can tell you that tires do not fail because of some RV-Magic. It comes down to actual Science.

Why do tires fail?

Simple question but as with almost every question about tires, the answer is not a one liner. This post. It's not short but it really isn't technical and I'm sure every reader will understand the concepts.

When looking at why tires fail we need to first exclude the obvious damage caused by external objects i.e. pot holes, curbs, chunks of metal, glass or rock or other trash and junk on the road. Second we'll exclude broken valve bodies, leaky valve cores, valve gaskets, extension hoses, and cracked rims.

What's left will be a structural failure which would almost 100 percent of the time will be either a detachment of different parts or components of a tire one from the other, OR a heat/fatigue failure in the mid sidewall due to driving with extreme low inflation.

Most tires are made up of 20 to 30 different components such as different steels (belts and bead are not the same steel) textiles such as Nylon, Polyester, Rayon, Aramid. Then there are the various different rubber components such as Tread, Sidewall, Innerliner, Steel skim, Wedge, Flippers, Chaffer, base tread, Inserts, and other bits and pieces.

Each component is selected for different reasons and contributes both advantages and disadvantages to the total. Each component must "stick" to its neighbors both in the un-cured and cured state. The interface between two different components is weaker than the individual components but it must hold together through millions of cycles over a 200F degree temperature range.

All would be just fine if a tire were made completely of inorganic materials such as metals, stone or even ceramics. There are numerous formulas for the strength and fatigue limits for these inorganic materials.

The issue with organics (wood, rubber or other materials made from oil such as plastics) is that their strength has a "T" or time function. If you build a bridge of stone, you can calculate the maximum load it can sustain and as long as the structure isn’t changed due to external damage its strength will be the same the day it’s built and 50 years later. If I build a pressure vessel of steel and put 150 psi in it and it doesn’t fail I would have every expectation for it to continue to hold that pressure for decades, again excluding external damage.

However, if I build a pressure vessel of organic materials (a tire) it might hold 150 psi the first day or maybe the first few days but at some point it can fail. This principle is not something many engineers think of today because they don’t design bridges of wood but 100 or 150 years ago when wood was common they learned that a bridge that was strong enough to hold a railroad train to drive across, it could fail if the train parked on the bridge.

I educated some fellow tire engineers about this “T” function when I proved, through lab experiments that it was possible to fail two tire with high pressure above its stated max 18 days after it was initially inflated.

The other condition that affects and changes the “T” in the equation is temperature. Organics experience constant change (loss) in properties as the temperature increases. The rate of change (aging) doubles about every 18 degrees F of increase in temperature, so it's not easy to calculate or predict the time it will take for an organic structure to fail unless you can control the temperature over time.

Bottom line. It’s a combination of temperature (heat) and time that causes tires to fail. A tire that spends its life in Flagstaff, Ariz., could probably last twice as long as a tire that spent its life in Phoenix if all other operating conditions were identical so even knowing the state where the RV was used isn't sufficient.

As the owner of an RV you can significantly affect one factor and that is the heat generated internal to the tire. When you run fast or overloaded or under-inflated you are running hotter. This means you are speeding up the rate of "aging" of your tires and can expect to see a tire fail before it wears out. You can also protect and shield your tires from the heat generated when in direct sunlight while your RV is parked. HERE you can read about an experiment I ran on the benefits of shielding your tires from Sun exposure.


Friday, April 8, 2022

Should I move from Load Range D to LR-E ?

 Following an RV Forum on trailers.

Originally Posted by NavyL
If the trailer came with Load Range D tires from the factory, and you replace them with Load Range E tires - I just don't see the need to run the Load Range E tires at the full 80 psi. Tire manufactures have inflation pressure v. weight charts for a reason.

My reply:

If you change from LR-D to LR-E (or from LR-C to LR-D) you will only get an increase in load capacity, or an increase in the Reserve Load, if you run higher than the Certification Sticker inflation psi. If you look at the Load & Inflation tables (they are almost all identical for ST and LT type tires) and look at your actual scale weights you can calculate your current Reserve Load and see what inflation you would need to get to a Reserve closer to 20% or 25%. Due to the unique forces ("Interply Shear") tires must tolerate on multi-axle trailers, I and other tire engineers suggest at least a 15% Reserve with 20% being better. On my personal RV I run 20 to 25% Reserve Load.

While we are talking about the certification label, I advise people to snap a picture of the label, or labels if more than one, along with a shot of the complete tire size and Load Range info on your tires PLUS the full DOT serial including the data code portion of the DOT serial. That way if the printing on the sticker fades or your tire gets cut and destroys itself you have a record so you can file a claim if you purchased "Road Hazard" warranty on your tires as I have, or need to check against a tire recall notice, you can answer the question of what size and Load Range you are currently running. Keep a picture of the Weight Slip too. Its a lot easier to have all this tire related info in one place on your phone.


Friday, April 1, 2022

Videos from Goodyear worth watching

 I would strongly recommend every RV owner watch the four videos from Goodyear.

Note: If you have problems running the videos you can try using VLC Video Player. Download for free HERE

 While they focus on RV applications, the information can still be informative and educational for anyone that owns a vehicle that has tires. Watching these will also give you a break from my run-on sentences in my sometimes over-detailed posts.

PS I know Tim Miller, the Goodyear Engineer in the video. While I never worked directly with Tim we did share some knowledge about tires in RV applications.